The present invention relates to a multi-fan assembly that has an electromagnetic device which generates a magnetic counter force to partially offset the revolution torque on the rotatable member thereby reducing the revolving speed of the multi-fan assembly round its main shaft. In other words, this invention uses an electromagnetic device as a braking unit to replace the mechanical device used in the case of U.S. Pat. No. 6,364,638 B1.
To avoid confusion during the course of this presentation, the word xe2x80x9crotationxe2x80x9d and xe2x80x9crevolutionxe2x80x9d are defined respectively as follows.
Rotation: the spinning of fan blades of the member fan rounds its own axis or the circular movement of a rotatable member moving round a central shaft.
Revolution: the circular movement of two or more member fans rotating about a common central main shaft.
A conventional multi-fan assembly is shown in FIG. 1. It generally includes a main shaft 41 fixed on the ceiling, a rotatable member 42 rotatably connected to the main shaft 41, two or more member fans 40 fixedly connected to the rotatable member 42, and two conductive members 43 fixedly connected to the rotatable member 42 to transfer power to the two member fans 40. Each member fan 40 has its own fan blades which rotate about its own axis. The self rotation of the member fans 40 creates a torque on the rotatable member 42 thereby causing the whole multi-fan assembly to revolve round its main shaft 41. The revolving speed of the multi-fan assembly is determined by the rotating speed of the member fans 40; when the member fans 40 are at low rotating speed, the multi-fan assembly also revolves slowly, and vice versa. The revolving of the multi-fan assembly creates an effect of total air management which gives air movement (wind flow) in all directions round its main shaft. Compared with the traditional fans which only gives a mono-directional air circulation, the revolvable multi-fan assembly is a much more efficient and powerful device for air circulation as well as cooling purpose.
The revolving speed, however, needs to be controlled to the extent that it does not disturb the normal performance of the member fans 40 while still possessing the benefit of the total air management effect. Therefore, in order to reduce the revolving speed a control device using a mechanical structure (U.S. Pat. No. 6,364,638 B1) has been developed in a multi-fan assembly as shown in FIG.2 which includes a gear operated speed reduction device 44 and a friction member 451 connected between the main shaft 41 and the rotatable member 42. Such mechanical control device has the following shortcomings in its role as a braking unit.
(1). It only works well when the member fans 40 are designed with one speed. If the consumers want a much lower or much higher wind flow from the member fans, the control quality for revolution becomes very poor. The reason is the mechanical control device which uses a pair of gear set 44 together with a friction member 451 (as shown in FIG. 2) can only create a fixed load to the rotatable member 42; at lowest fan speed the revolution torque generated by the rotation of the member fans 40 is always not sufficient to overcome the fixed load of the mechanical control device, therefore, the multi-fan assembly often stands still with no revolution effect. On the other hand when the member fans 40 are set at the highest speed, the revolution torque becomes too large for the mechanical control device to control and consequently the fan assembly revolves uncontrollably fast making the total air management effect meaningless. According to the actual performance data, when the member fans 40 are set at a speed of 1500 rpm, the revolution of the two member fans round the main shaft is 120 rpm and when the speed of member fans is set below 400 rpm, the revolution effect doesn""t occur. The total air management effect is useful only if the revolution speed can be controlled within an optimum range of 5 rpm to 25 rpm.
(2). The friction member 451 and the gears of the speed reduction device 44 unavoidably make different degree of noises in the course of revolution.
(3). The wearing of the parts of the friction member 451 and the gears of the speed reduction device 44 requires costly replacement and maintenance.
The present invention intends to provide a new technology that can effectively manage the revolution speed of a multi-fan assembly at user""s discretion regardless of the rotation speed of the member fans meanwhile it also remedies the other shortcomings as described above.
In accordance with one aspect of the present invention, there is provided a multi-fan assembly which comprises a central main shaft, a rotatable member rotatably connected to the central main shaft, two conductive members and two fans connected to the rotatable member. Two conductive pieces are connected to the central main shaft and the two conductive members receive power from the two conductive pieces and transfer power to the two member fans. Additionally an electromagnetic control unit is built in the space between the central main shaft and the rotatable member. The control unit includes a magnetic induction mask fixedly received in a space in the rotatable member, a magnetic flux coil fixed to the central main shaft and snugly surrounded by the magnetic induction mask, and an electric current control unit also connected to the central main shaft. The magnetic induction mask is composed of array of mutually insulated inductible metal bars parallelly fixed along the inner circular wall of the magnetic induction mask. The magnetic flux coil is composed of stack of silicon steel sheet coiled with copper wires. The electric current control unit controls the current going into the magnetic flux coil and consequently the magnetic induction mask and the rotatable member together are magnetically attracted by silicon steel of the magnetic flux coil thereby creating a magnetic counter force to partially or completely offset the revolution torque generated by the rotation of the two member fans. The strength of magnetic counter force is determined by the level of current provided by the current control unit. Therefore, the revolving speed of the multi-fan assembly round its central main shaft can be managed at user""s discretion by simply adjusting the level of current going into the magnetic flux coil.